This PR fixes GH Issue #254.

With this fix, both the mulitple submodels as array,
and a single submodel can be imported to the package explorer.

This way the functionality is also interoperable
with the package explorer.

{
    "submodels": [
        {
            "modelType": "Submodel",
            "id": "https://acplt.org/Simple_Submodel"
        }
    ]
}

Single Submodel:

{
    "modelType": "Submodel",
    "id": "https://acplt.org/Simple_Submodel"
}